This invention relates to composite materials. More specifically, this invention relates to composite friction materials which exhibit high, stable, coefficients of friction over a wide temperature range.
The elastomeric materials heretofore proposed for use as materials have generally proven to be unsatisfactory when exposed to high ambient working temperatures such as encountered, for example, in clutch and brake applications in heavy duty service vehicles. Typically, such materials have been based on heat-hardenable resins such as phenol-aldehyde resins which tend to heat-decompose under the high peak and bulk temperature conditions created by the sustained and/or heavy loading forces experienced in the clutch and brake systems of these vehicles while operating. As a result of this decomposition, the physical properties of these materials typically deteriorate, and the consequent disintegration of the material and dispersal of the products of heat decomposition generally interfere with the functioning of the friction unit. Furthermore, many times after friction material comprising a partially heat-decomposed heat-hardenable resin has cooled, the material will exhibit inconsistency with respect to coefficient of friction.
These conditions, as well as other problems associated with these and similar friction materials, result in a loss of efficiency in the friction unit and unreliability in the service vehicle, which is highly undesirable.
Many attempts have been made to obviate the problems associated with the elastomers in general use as friction material bases. Many different resins have been experimented with, in attempts to obtain a friction material which possesses a high, stable coefficient of friction over a wide temperature range. Modification of the heat-hardenable resins with other polymeric materials has been attempted. Many of these friction material formulations have not performed well. Other formulations have required multi-step procedures which are costly in terms of labor and frequently in terms of the material used in these formulations.
Importantly, also, many of these known friction materials require a bonding agent to affix them to the backing plate or "core" portion of the friction element. This requirement severely restricts the scope of the molding methods and mold configurations employable in forming these friction elements. In injection molding, for example, the bonding agent is subject to scuffing during the molding process, which deactivates or destroys the bond and renders this molding process useless with these friction elements. In general, where bonding agents must be utilized, only compression molding and relatively simply mold configurations can be employed in the process of molding the friction element.
In order to obtain a friction material with a usefully high coefficient of friction which is stable over a wide temperature range, the industry has most usually used nonresilient inorganic friction materials such as sintered bronze. Although the friction characteristics of this and similar metallic materials have been generally satisfactory under high temperature conditions, the high modulus or lack of resiliency of these materials and their resultant inability during operation to conform to the friction element mating surface and absorb adequate energy result in relatively high wear rates and shortened life. Furthermore, great care must be taken in the type and viscosity of oil used in conjunction with such friction materials during use to ensure that the desired coefficient of friction is not impaired.
It has been determined by actual tests that the bronze clutch material, when employed in the high load oil-cooled clutch environment of a transmission for a heavy duty earthmoving vehicle, exhibited some measurable difference in its frictional properties as a function of viscosity grade of the lubricant. It was theorized that at least a part of the energy absorption during the engagement cycle between the friction plate and the reaction plate was through shear of the fluid as well as the boundry lubrication or intimate contact between the clutch plate and reaction plate. It was further recognized that great advantage could be obtained by maintaining the oil film thickness to a minimum dimension in order to permit more of the energy of engagement to be absorbed by shear in the oil film and that a further advantage would be gained by sustaining such thin film in the order of a few microinches during a longer portion of the engagement cycle. Several well known principles of fluid mechanics were considered to aid in this development. For example, the oil film thickness between a rotary friction plate and a stationary substantially flat reaction plate is a function of the radius of curvature or size of the protuberances or asperities in the friction material under a given load. Accordingly, the smaller the asperities in the surface of the friction plate, the thinner the oil film thickness. It is also known that the thinner the oil film thickness, the greater rate of shear and, therefore, the greater shear force and energy that the oil film is capable of absorbing. Accordingly, a great plurality of such asperities in the surface of the friction plate produces greater fluid wedging effect of the oil film as it is squeezed between the multitude of asperities and the reaction plate to produce a substantial drag on the friction plate. Furthermore, the greater the amount of oil film in thin film shear, as discussed above, the greater the energy absorption. Accordingly, the substantially large number of asperities in the surface of the friction plate results in a greater amount of oil being retained over the entire surface area of the plate due to the trapping effect and cavitation of oil on the trailing sides of the asperities over the surface area between the friction material plate and its mating reaction plate. It is further recognized that the greater the real viscosity or internal resistance to flow of fluid in contact between the plates, the greater the energy absorption obtainable in the thin film shear. Such real or developed viscosity during the engagement cycle is, of course, enhanced by the substantial number of asperities through which the engagement pressure is transmitted to the oil film in shear. It is also known that the greater the relative motion between the friction plate and the reaction plate, the greater the shear and, again, the greater energy absorption obtained. In using the above-discussed simple fluid mechanics principles, the problem was prsented of how best to maximize energy absorption between a pair of friction plates through a fluid film with the greatest reliability and over substantially the entire range of the engagement cycle.
Accordingly, it is an object of the present invention to provide an improved friction coupling which utilizes an improved friction material capable of cooperating with a fluid for producing a thin fluid film between it and the reaction member wherein a substantial portion of the energy of plate engagement is absorbed in shear of the fluid film for greater wear resistance, temperature stability and a higher coefficient of friction than heretofore obtainable.
Another object of the present invention is to provide an improved friction coupling in which the friction plate is formed of a relatively soft elastomeric material and has intermixed therewith a plurality of relatively high modulus asperities in the surface thereof which cause a hydrodynamic wedging of the fluid to establish a load absorbing film of separating fluid between the friction plates for maximizing the absorption of energy during plate engagement.
It is a further object of this invention to provide a friction coupling utilizing a friction material composition with high dynamic and static coefficients of friction over a wide temperature range.
It is a further object of this invention to provide a friction coupling utilizing a friction material composition which can readily be bonded to a metal core material.
It is an additional object of this invention to provide a friction coupling utilizing a friction material composition which can be injection or compression molded, and which can be molded in conjunction with complex mold configurations.
It is also an object of this invention to provide a friction coupling utilizing a conformable, long-wearing friction material composition with a high, stable coefficient of friction over a wide temperature range.
Other objects and purposes of this invention will be apparent to those skilled in the art from the disclosure contained herein.